Process for manufacturing LLDPE polymers

ABSTRACT

PCT No. PCT/FI96/00240 Sec. 371 Date Feb. 10, 1998 Sec. 102(e) Date Feb. 10, 1998 PCT Filed Apr. 30, 1996 PCT Pub. No. WO96/34895 PCT Pub. Date Nov. 7, 1996Process for manufacturing ethylene copolymers by polymerizing ethylene and a minor amount of C3-C6 alphaolefins in particle form in slurry reactor in the presence of an ethylene polymerizing catalyst. According to the invention the polymerization is carried out in propane diluent by using a metallocene catalyst activated with an alumoxane compound.

The invention relates to a process for manufacturing ethylenecopolymers, especially linear low density polyethylene polymers having anarrow molecular weight distribution and a narrow compositiondistribution.

The prior art includes several various processes for manufacturingpolyethylene. Thus typical processes are gas-phase and slurry processes.In the latter process the polymerization is typically performed in areaction medium or diluent, which is formed by isobutane, pentane,hexane or some other saturated aliphatic hydrocarbon. The polymerizationis often performed in the presence of a Ziegler-Natta catalyst at anelevated temperature. In addition of monomers, hydrogen is often used asa modifier in the polymerization for affecting the molecular weight ofthe polymers achieved.

The production of LLDPE materials with traditional Ziegler-Nattacatalysts in slurry process is difficult, because the solubility of thepolymer is too high due to broad molecular weight distribution andcomonomer distribution achieved by traditional Z-N catalysts. Thislimits strongly the reactor operating temperatures and hence also theproductivity of the catalyst. Due to solubility limitations LLDPEmaterials are normally produced commercially in gas-phase reactors andsolution process.

Recent developments in the field of olefin polymerization catalystsinclude metallocene catalysts, which comprise metallocene compounds oftransition metals together with alumoxane compounds. These catalystshave been suggested to be used as a homogenous system or deposited ontocarrier, for example inorganic oxide carriers. Thus these catalyststypically comprise as a procatalyst component a metallocene compound,for example bis(cyclopentadienyl)titanium dialkyl orbis(cyclopentadienyl)zirconium alkonyl or chlorides thereof, and anactivator component, which typically is alumoxane or an ionic activator.

WO94/21691 describes a process, where ethylene and C₃ -C₈ alpha-olefinare polymerized in a slurry in the presence of a metallocene catalyst.The polymerization is carried out in a stirred tank reactor in an alkanesolvent selected from isobutane, pentane or heavier alkanes. Althoughthe publication states that a polymerization temperature of 70-100° C.can be used, the examples of the publication are all carried out in thetemperature of 70° C.

Also this process describes only a batch process. There is no indicationthat a process could be carried out as a continuous process.

It has been found that metallocene catalysts have a property ofproducing polyethylene having much more narrow molecular weightdistribution and much more homogenous comonomer incorporation, whichmeans that all polymer chains will have an equal amount of comonomer andit will be distributed homogenously. Therefore it would be expected thatthey could be applied for manufacturing in slurry process such LLDPEpolymers which would have a relatively narrow molecular weightdistribution and narrow composition distribution.

It has also been observed that in a standard slurry process polymersolubility into the diluent in order to produce low density productsrequires temperatures which are rather low and this results in lowproduction capacity and long and expensive transition periods. Also theoperation in standard slurry process has indicated that the productivityof the metallocene catalysts decreases when the reactor temperature isincreased. According to the invention it has surprisingly been found outthat when the temperature is further increased the productivity of thecatalyst can be improved.

Thus the object of the invention is a process for manufacturing ethylenecopolymers such as medium weight polymers and linear low densitypolyethylene by polymerizing ethylene and a minor amount of C₃ -C₈alphaolefins in particle form in slurry reactor in the presence of anethylene polymerizing catalyst. The process of the invention ischaracterized in that the polymerization is carried out in propanediluent in a temperature above 80° C. and that said catalyst is ametallocene catalyst activated with an alumoxane compound.

The use of higher reactor temperature increases the reactivity of thecomonomer thus reducing the required amount of the comonomer in thereactor to achieve target density compared to the operation at lowertemperatures.

The molecular weight of the produced polymer decreases when reactortemperature is increased. This could be compensated by using higherethylene concentration in the reactor but it may cause process problemsin the traditional slurry processes in form of bubble formation andcavitation in the reactor. According to a preferable embodiment of theinvention the use of supercritical conditions (where no phase separationbetween gas and liquid exists) offers an additional benefit ofcontrolling the produced molecular weight by ethylene concentration. Ithas been found out that even in a traditional slurry process themolecular weight can be controlled to some extent by ethyleneconcentration in the reactor. The use of supercritical conditions allowsmuch wider control range of the molecular weight.

The excellent polymer morphology of the products produced with themetallocene catalysts together with the low polymer solubility into thediluent and relatively low diluent density, especially in thesupercritical conditions, result on very good settling properties of thepolymer and thus efficient reactor operation (i.e. diluent flow into thereactor can be minimized.).

The use of high polymerization temperatures reduces polymer solubilityinto the diluent enabling the production of low density products inhigher reactor temperatures.

Due to the above mentioned benefits the operation efficiency can bedrastically improved by operating the reactor using propane diluent,especially in supercritical conditions: higher temperature means highercatalyst productivity, better comonomer reactivity, faster grade changesand efficient operation.

The polymerization is carried out at a temperature, which is above 80°C., preferably above 85° C. According to one embodiment thepolymerization is carried out at a temperature and at a pressure, whichare above the corresponding critical temperature and pressure points ofthe mixture formed by ethylene, comonomer, diluent and optionallyhydrogen.

The process according to the invention is carried out by continuousslurry process by using propane as reaction medium and a loop reactor asa polymerization reactor. The catalyst, propane, ethylene and comonomerand optionally hydrogen, are fed continuously into the reactor. Thereaction mixture is continuously stirred or circulated through thereactor, whereby a slurry of polyethylene and hydrocarbon is formed.

According to one embodiment of the invention the temperature is selectedso that it is above the critical temperature of the reaction mixture butbelow the melting or softening point of the product. Therefore thetemperature is selected between 95-110° C., preferably between 96-105°C.

The invention is not limited for the single reactor operation. Thecontrol of molecular weight of the polymer by ethylene concentration canbe fully utilized when operating two or more reactors in seriestargeting the production of broad MWD/bimodal polymers. In thisparticular case the high molecular weight fraction can be produced byhaving very high ethylene concentration in one of the reactors and thelow molecular weight fraction by having low ethylene and high hydrogenconcentration in the other reactors.

The density of the polymers is controlled by addition of conomomers tothe polymerization. Suitable comonomers to be used according theinvention are C₃ -C₈ olefins, preferably butene or 1-hexene.

As a catalyst a metallocene type catalyst is used. As metallocenecompounds it is possible to use any kind and type of metallocene. Thussuitable metallocene compounds are those which have a formula (Cp)_(m)R_(n) MR'_(o) X_(p), where Cp is an unsubstituted or substituted and/orfused homo or heterocyclopentadienyl, R is a group having 1-4 atoms andbridging two Cp rings, M is a transition metal of group 4, 5 or 6 in thePeriodic Table of Elements (IUPAC, 1985), R' is C₁ -C₂ hydrocarbyl orhydrocarboxy group and X is a halogen atom, wherein m is 1-3, n is 0 or1, o is 0-3 and p is 0-3 and sum n+o+p corresponds the oxidation stateof the transition metal M. The transition metal M is preferablyzirconium, hafnium or titanium, most preferably zirconium. Examples fromsuitable metallocene compounds are, among others),bis(n-butycyclopentadienyl)zirconium dichloride andbis(indenyl)zirconiumdichloride.

The polymerization activity of the catalyst component described abovecan be increased by known activator compounds, such as alumoxanecompounds. One method is to add the alumoxane compound to themetallocene containing catalyst compound. In that case the alumoxanecompound is preferably added by impregnation method, in which a solutionof alumoxane compound is impregnated into the catalyst component. Theamount of such solution is preferably not greater than the total freepore volume of the catalyst compound already containing the metallocenecompound. After impregnation the solvent can be removed for example byevaporation. Another method for applying activator compounds is to addit straight into the polymerization reactor along with the metallocenecontaining catalyst component.

Suitable activators are for example alumoxane compounds having a formulaR--(Al(R)--O)_(n) --AIR₂ or (--Al(R)--O--M)_(m), where n is 1-40, m is3-40 and R is a C₁ -C₈ alkyl group. Preferably R is a methyl group.

The support or carrier material used in the method according to theinvention may be any porous, substantially inert support, such as aninorganic oxide or salt. In practice the support used is preferably afine-grained inorganic oxide such as an inorganic oxide of an element ofGroup 2, 13 or 14 in the Periodic Table of Elements (IUPAC, 1985), mostpreferably silica, alumina or a mixture or derivative of these. Otherinorganic oxides which can be used either alone or together with silica,alumina or silica-alumina, are magnesium oxide, titanium dioxide,zirconium oxide, aluminum phosphate etc.

The support used in the method is preferably dry. In general, metaloxide supports also contain surface hydroxyl groups which may react withmetallocene or alumoxane. Therefore the support can be dehydrated ordehydroxylated before use. Such treatment may be either a thermaltreatment or a reaction between the surface hydroxyl groups of thesupport and a reagent contacted with it.

Preferable support materials to be used according to the invention areporous silica or alumina carriers. The pore volume is not critical andcan be varied within rather wide limits, but normally in commercialsupport materials the pore volume is preferably approx. 0.9-3.5 ml/g.

The process according the invention has several advantages over theprior art slurry processes. By using metallocene catalysts a more narrowmolecular weight distribution and more homogenous comonomer distributionis achieved, which is desirable in LLDPE products. By using propanediluent and especially a polymerization temperature and pressure whichare above the critical points of the reaction mixture, the polymerizingactivity of the catalyst can surprisingly be increased. The loweringeffect on the molecular weight because of high temperatures can beeliminated by using high ethylene concentrations, which in the inventiondoes not cause processing problems, such as bubble formation. Furtherthe molecular weight can be regulated by varying the amount of hydrogenadded into the polymerization. Further the desired low densities can beachieved by using lower amounts of comonomer in the polymerization.

The invention is further illustrated by accompanying examples wheremetallocene catalysts were used in the polymers under subcritical andunder supercritical conditions.

Example 1

Catalyst Preparation

26.78 kg of silica calcined at 600° C. for 4 hours was placed in 150 dm³reactor equipped with an effective stirrer. A complex solutioncontaining MAO and metallocene was prepared as follows: 295 g ofbis(n-butyl-cyclopendienyl)zirconium dichloride was dissolved in 6.5 kgof dried and deoxygenated toluene. To this metallocene solution 29.1 kgof 30 wt-% MAO in toluene was added and mixed. This solution containigMAO and metallocene was then added to the silica and after adding thestirring was continued for two hours before evaporation of toluene wasstarted. The Al and Zr-contents of the final catalyts were 8.8 and 0.12w-% respectively. The toluene content of the final catalyst was 2.4-%.

Polymerizations

Polymerizations were done in 2 dm³ stainless steel reactor equipped withpaddle stirrer. 1 dm³ of dried and deoxygenated propane was firstintroduced into the reactor at room temperature. After adding thecatalyst the reactor was heated up to desired temperature. Then ethyleneand hexene and optionally hydrogen was fed simultaneously to thereactor. The partial pressure of the monomer and hydrogen was kept bycontinuously feeding ethylene to the reactor. After 1 hour thepolymerization was stopped rapidly by venting off the ethylene andcooling down the reactor.

Example 1 (comparison)

105 mg of catalyst described above was used in the polymerization. Thepolymerization temperature was 70° C. and ethylene partial pressure was10 bar. 60 ml of hexene was used in the polymerization. After 1 hour 147g of ethylene-hexene copolymer was obtained. The molecular weight of thepolymer was 100,000 and the molecular weight 2.65 determined with GPC.The density of the copolymer was 930.8 kg/M³. Hexene content in thepolymer was 2.5 w-% (FTIR).

Example 2 (comparison)

Polymerization was done as in Example 1, but 95 mg of catalyst was usedand the polymerization temperature was raised to 80° C. 88 grams ofethylene-hexene copymer was obtained. The density of the polymer was927.7 kg/m³. The hexene content in the polymer was 2.9 w-% determinedwith FTIR.

Example 3

Polymerization was carried out as in Example 1, except that 263 mgcatalyst was used and the polymerization temperature was increased to85° C. Ethylene partial pressure was 5 bar and 20 ml butene was added.After the reaction has proceeded for 52 minutes, 312 g of polymer wascollected, with MFR₂ 1,7 and density 926.4. The butene content of thepolymer was measured to be 2.9 w-%.

Example 4

Polymerization was carried out as in. Example 3, except that thepolymerization temperature was increased to 90° C. Ethylene partialpressure was 5 bar and 20 ml butene was added. After the reaction hasproceeded for 40 minutes, 326 g of polymer was collected, with MFR₂ 1,5and density 922.7. The butene content of the polymer was measured to be3.7 w-%.

Example 5

Polymerization was done as in Example 1, but 205 mg of catalyst was usedand the polymerization temperature was raised to 96° C., ethylenepartial pressure was only 5 bar and 30 ml of hexene was used. 379 gramsof ethylene-hexene copolymer was obtained with molecular weight of110,000 and MWD of 2.50. The density of the polymer was 925.1 kg/m³ andhexene content in the polymer was 3.5 w-% determined with FTIR.

The examples are illustrated in the following table:

    __________________________________________________________________________                                      Co-                                                  C.sub.2 -                                                                        Co-                      monomer                                              monomerressure                                                                     Activity                                                                             M/W                                                                                       commentity                                Ex.   ° C.                                                                                   kg PE/g/h                                                                      g/mol                                                                             M.sub.w /M.sub.n                                                                 kg/m.sup.3                                                                        w-%                                         __________________________________________________________________________    1   70  10  60   1.40 100000                                                                             2.65                                                                             930.8                                                                             2.5                                         comp.               hexene                                                    2       80                                                                                          0.93                                                                              n.a.                                                                              n.a.                                                                                 2.97                                     comp.                                                                         3       85                                                                                          1.40                                                                              n.a.                                                                              n.a                                                                                  2.94                                                         butene                                                    4       90                                                                                          2.3                                                                                   n.a.                                                                                 3.77                                                         butene                                                    5       96                                                                                          1.85                                                                              110000                                                                          2.5                                                                                    3.5.1                                                        hexene                                                    __________________________________________________________________________

These examples clearly show that it is possible to achieve betteractivities of the catalyst and lower density when temperature isincreased over 80° C., and more comonomer in the product is achievedwith lower comonomer content in the polymerization.

We claim:
 1. A process for manufacturing ethylene copolymers whichcomprises polymerizing ethylene and a minor amount of C₃ -C₆ alphaolefins in particle form in a slurry reactor in the presence of anethylene polymerizing catalyst, wherein polymerizing is carried out in apropane diluent containing ethylene, said C₃ -C₆ alpha olefin andoptionally hydrogen, at a temperature above the critical temperature ofthe mixture, but below the softening temperature of the polymer and at apressure above the critical pressure of the mixture and said catalyst isa metallocene catalyst activated with an alumoxane compound, saidethylene copolymers having a density of between 910-928 kg/m³. 2.Process according to claim 1, which is carried out continuously in atleast one loop reactor.
 3. Process according to claim 1, wherein saidcatalyst is formed from a metallocene having a formula (Cp)_(m) R_(n)MR'_(o) X_(p), where Cp is an unsubstituted or substituted and/or fusedhomo or heterocyclopentadienyl, R is a group having 1-4 atoms andbridging two Cp rings, M is a transition metal of group 4, 5 or 6 in thePeriodic Table of Elements (IUPAC, 1985), R' is C₁ -C₂ hydrocarbyl orhydrocarboxy group and X is a halogen atom, wherein m is 1-3, n is 0 or1, o is 0-3, p is 0-3 and sum n+o+p corresponds the oxidation state ofthe transition metal M.
 4. Process according claim 3, wherein M iszirconium, hafnium or titanium.
 5. Process according to claim 4, whereinsaid metallocene compound is bis(n-butylcyclopentadienyl)zirconiumdichloride or bis(indenyl)zirconiumdichloride.
 6. Process according toclaim 3, wherein the catalyst is activated with an alumoxane compoundhaving a formula

    R--(Al(R)--O).sub.n --AlR.sub.2 or (--Al(R)--O--).sub.m,

where n is 1-40, m is 3-40 and R is a C₁ -C₈ alkyl group.
 7. Processaccording to claim 3, wherein said catalyst is supported on a porousorganic or inorganic carrier material.
 8. Process according to claim 1,wherein polymerization is carried out at a temperature of 95-110° C. andat a pressure above 50 bar.
 9. Process according to claim 1, wherein thepolymer product has a M_(w) /M_(n) of 2-3.